Technical Field
The present disclosure relates to a diode with reduced recovery time. In particular, the present diode is suited to being used, for example, in the case of applications potentially subject to the so-called current recirculation phenomenon. More in general, the present diode is suited to being used in the case of applications that envisage subjecting the diode to fast voltage variations.
Description of the Related Art
As is known, the so-called current recirculation phenomenon presents, for example, when an output electronic stage is connected to a (desired or parasitic) inductive load.
For instance, FIG. 1 shows a control stage 2, which includes a first transistor 4 and a second transistor 6, as well as a first diode D1, a second diode D2, a third diode D3, a fourth diode D4, a fifth diode D5, and a sixth diode D6.
In detail, the first transistor 4 is a power MOSFET of a PMOS type and includes the fifth diode D5. The cathode and the anode of the fifth diode D5 are connected to the source terminal and to the drain terminal, respectively, of the first transistor 4.
Furthermore, the source terminal of the first transistor 4 is set at a first (positive) supply voltage V+, whereas the drain terminal is connected to the anode of the third diode D3, the cathode of which forms a node N.
The second transistor 6 is a power MOSFET of an NMOS type and includes the sixth diode D6. The cathode and the anode of the sixth diode D6 are connected to the drain terminal and to the source terminal, respectively, of the second transistor 6.
Furthermore, the source terminal of the second transistor 6 is set at a second (negative or zero) supply voltage V−, whereas the drain terminal is connected to the cathode of the fourth diode D4, the anode of which is connected to the node N.
The anode and the cathode of the first diode D1 are connected to the node N and to the source terminal of the first transistor 4, respectively. Further, the cathode and the anode of the second diode D2 are connected to the node N and to the source terminal of the second transistor 6, respectively.
The node N is electrically connected, for example, to a metal pad 10. In this connection, the third and fourth diodes D3 and D4 serve to prevent, in the case where the first transistor 4 and/or the second transistor 6 are off, a possible signal coming, through the metal pad 10, from the outside world, from traversing the fifth and sixth diodes D5, D6.
Once again as regards the metal pad 10, it is electrically connected to a load formed by a series circuit, which in turn includes an inductor L and a resistor R.
In use, the first and second transistors 4, 6 are controlled, through the respective gate terminals, so as never to be simultaneously in conduction. This having been said, assuming that the first transistor 4 is in conduction, current does not flow in the first diode D1. Further, the second diode D2 sustains the voltage present between the voltage on the node N and the voltage V−, but no current flows in it either. In these conditions, a certain current flows in the load, and thus in the inductor L and in the resistor R.
Then, while the second transistor 6 remains off, the first transistor 4 is off. In these conditions, the inductor L tends to maintain the current that traversed it when the first transistor 4 was on. This current is, however, delivered now by the second diode D2, for a certain period of time. Next, the second transistor 6 is turned on and brings the voltage of the node N to a value approximately equal to V. The current continues to flow in the second diode D2 until the inductor L has exhausted the energy accumulated during the previous conduction step. Once said energy is exhausted, the inductor L is traversed by a current having an opposite direction with respect to the previous conduction step, this current further flowing through the fourth diode D4 and the second transistor 6; in these conditions, the second diode D2 starts to turn off.
This having been said, if before the second diode D2 is completely off (i.e., it is without accumulated charge), the second transistor 6 is turned off and then the first transistor 4 is turned back on, the voltage on the node N rises. In other words, the first transistor 4 tends to force the second diode D2 to operate in reverse mode. However, the second diode D2 is not yet off and must in any case sustain the voltage present on the node N; in these conditions, the second diode D2 may be subject to failure since the voltage across the second diode D2 may be sustained only by the portions of the second diode D2 that are without carriers.
A qualitative example of the plot of the current in the second diode D2 is illustrated in FIG. 2, where V denotes the voltage on the node N, which at a first instant t1 switches from a value approximately equal to V+ to a value approximately equal to V−, and at a subsequent second instant t2 switches from the value approximately equal to V− to the value approximately equal to V+. Furthermore, FIG. 2 also shows the plot of the current (designated by I) that flows in the second diode D2, on the hypothesis that, at an instant t0 prior to the first instant t1, the first transistor 4 is turned off, and that at said instant t0 the current I is equal to a value ID. This having been said, between the instant t0 and the second instant t2, the current I decreases; however, at the second instant t2, i.e., at the instant when, after the second transistor 6 has been turned off, the first transistor 4 is turned back on, the current I is not completely zero, or in any case (case not illustrated) the second diode D2 is not completely depleted; i.e., it is not completely off. For this reason, following upon the second instant t2, the current I decreases sharply, on account of the charge still stored in the second diode D2, until it reverses its direction. In particular, at a third instant t3, the second diode D2 is traversed by a reverse current equal to IR, which then vanishes at a fourth instant t4 (the exhaustion curve is illustrated in FIG. 2 in a particularly simplified way, for the sole purpose of facilitating understanding). In practice, it may be noted that between the second instant t2 and the third instant t3 failure of the second diode D2 may occur. Similar considerations apply, more in general, also in the case where a so-called current recirculation does not occur, but a diode that is not completely off is in any case subject to sudden voltage changes.
In general, the problem of switching-off of a forward biased diode is strictly correlated with the so-called recovery time, which in turn depends, among other things, upon how the charge is stored within the diode. The longer the recovery time, the higher the likelihood of failure of a diode, when used in applications of the type described above.
This having been said, a treatment of the recovery processes in semiconductor power rectifiers is developed, for example, in “Reverse Recovery Processes in Silicon Power Rectifiers”, by H. Benda et al., Proceedings of IEEE, vol. 55, no. 8, August 1967.
In order to reduce the recovery time of a diode, some solutions have been proposed, which prove effective in the case of discrete diodes. In particular, it has been proposed to introduce recombination centers in the semiconductor to accelerate recombination of the excess carriers, i.e., absorption of the charge accumulated during conduction. For this purpose, it is possible to carry out an implantation of ions of heavy metals, or else radiate the semiconductor body with high-power radiation. In this way, the lifetime of the carriers is reduced. These solutions prove particularly advantageous in the case of discrete diodes. However, they are substantially impracticable in the case of multi-component devices integrated in dice, for example with bipolar-CMOS-DMOS (BCD) technology, which, as is known, is a technology (also known as “smart power”) that enables integration of CMOS and DMOS transistors in a same bipolar die. In fact, in the case of the implantation of metal ions, the latter tend to spread, contaminating the entire semiconductor wafer. In the case, instead, of high-energy radiation, it leads to an increase in the leakage currents with the device off.